Humans dominate the animal world. Whether hunting or competing for limited space and resources, we are the planet’s superpredator. Other animals seem to understand this, avoiding people if they can help it. But as the human population expands, it is getting harder for other creatures to find somewhere to hide during the day. Now new findings indicate mammals around the world have come up with another strategy: They are becoming nocturnal. Exactly what this bizarre shift means for the future of individual species—and entire ecosystems—is unknown.

In a paper published Thursday in Science, researchers analyzed 76 previous scientific studies about human impact on mammal activity. These studies had captured the 24-hour movements of medium- and large-bodied mammals, ranging from opossums to African elephants, encompassing 62 species across six continents. Although these past studies had discovered specific cases in which human disturbance had changed mammals’ behavior, “we set out to see just how widespread the effect was, and to try to quantify [it],” says Kaitlyn Gaynor, a PhD student in wildlife ecology at the University of California, Berkeley, and lead researcher for the new study. Gaynor and her team used the 76 studies to compare animals’ activity during the day and night in areas of high human disturbance (meaning anything from hunting or farming to hiking and other outdoor recreation) and low human disturbance (relatively natural conditions).

Wild Boar (Sus scrofa) looking for food near garbage in Barcelona, Spain. Credit: Laurent Geslin

The team’s analysis showed mammals are becoming an average of 1.36 times more nocturnal due to high human disturbance. Gaynor explains what that means: “If you have an animal that typically splits its activity 50–50 between the day and the night, that means it would then become 68 percent nocturnal [due to humans]—so that’s pretty striking.” The effect was particularly large in a number of species typically only active during the day. The results are likely not a huge surprise to researchers working in the field, says Carolyn Mahan, a professor of biology at The Pennsylvania State University Altoona who was not involved in the study. But “you can start to see the pattern across a larger scale—to see these trends are occurring across continents, across different species and across different human activities.”

For example, leopards in the Central African nation of Gabon are 46 percent nocturnal in areas without bushmeat hunting, but 93 percent nocturnal where the practice is common. In Poland wild boars go from 48 percent nocturnal in natural forests to 90 percent nocturnal in urban areas. Even activities people consider relatively innocuous, such as hiking and wildlife viewing, strongly affected animals’ daily rhythms. Brown bears in Alaska live 33 percent of the day nocturnally when humans stay away, but that number goes up to 76 percent for bears exposed to wildlife-viewing tourism. “We think that we're leaving no trace often when we’re outdoors, but we can be having lasting consequences on animal behavior,” Gaynor says.

This is not the first time mammals have resorted to living at night; during the time of dinosaurs, they were also nocturnal. “Dinosaurs were this ubiquitous, scary force, and only after the extinction of dinosaurs did mammals emerge into the daylight,” Gaynor says. “And now humans have taken over as the new ubiquitous, scary force and are pushing other animals back into the night.” Gaynor believes this evolutionary history may be helping modern mammals adapt to today’s human pressures and become more nocturnal.

Although scientists do not yet know the consequences of such a dramatic lifestyle change for individual species, they suspect it may hurt those highly adapted to finding food, communicating with others or escaping predators while the sun is shining. They might not be able to do those things well at night, which would ultimately hurt their chances of survival and reproduction. “In ecological terms my gut feeling is that the majority of effects would be negative,” says Thomas Mueller, a professor for movement ecology and biodiversity conservation at Goethe University Frankfurt, who did not take part in the work.

Perhaps even more alarming is the cascade of effects that could occur in the wider ecosystem as animals switch from day to night. “Patterns of competition and predator–prey interactions might change with the nocturnal behavioral changes,” Gaynor says. If one species—say a top predator—starts hunting at night and goes after different types of prey, it will likely have innumerable trickle-down consequences for everything along the food chain. Scientists have already observed such an ecological shift in California’s Santa Cruz Mountains, where coyotes have become more nocturnal in response to hikers. Researchers found these coyotes have started to alter their diets from daytime prey, such as squirrels and birds, to nocturnal prey, such as rats and rabbits.

Exactly how ecological communities will change, and whether it will be for better or worse, requires further study. “I think this is the next frontier for research,” Gaynor says. Some nighttime shifts may benefit both animals and humans, she notes. For instance, tigers in Nepal are avoiding potentially deadly conflicts with people as they become more nocturnal.

Studies like this one will eventually help conservation managers make better decisions about how to protect ecosystems, although it is not clear yet what strategies they should adopt. “We’ll need to understand local dynamics to really understand how we should be changing management of wildlife populations or human activities,” Gaynor notes. But her team’s paper offers one potential approach: creating landscapes with “temporal zoning,” where people limit disturbances at certain hours. “We might have to manage the timing of human activities,” she says, “so that we leave some of the daylight for other animals.”